Perdue, a former governor of Georgia, will sit out President Trump's State of the Union address at the U.S. Capitol, waiting for the worst.

For decades, presidential administrations have selected one Cabinet member to stay away from State of the Union speeches attended by the president and his staff, in case disaster strikes and wipes out the line of presidential succession.

During the speech, the designated survivor is taken to a safe, secure, undisclosed location. The identity of the official is kept secret until just before the speech, when his or her absence would be apparent.

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When Trump spoke to a joint session of Congress in February of 2017, Veterans Affairs Secretary David Shulkin sat out the event as the designated survivor.

The tradition harkens back to the dawn of Cold War, amid fears of a nuclear exchange with the Soviet Union.

Its roots are in the Presidential Succession Act of 1947, which outlines who would assume the presidency in the case of a catastrophic event such as a nuclear attack.

After the vice president, the speaker of the House, and the president pro tempore of the Senate, the line of succession goes to cabinet secretaries in the order they were created, beginning with the secretary of State and ending with the secretary of Homeland Security.

One member of Trump’s cabinet cannot serve as a designated survivor: Secretary of Transportation Elaine Chao. She was born in Taiwan and does not meet the constitutional requirements to serve as president.

The selection of a designated survivor was relatively routine and unremarkable until the terrorist attacks of Sept. 11, 2001, when the practice took on new urgency.

Hollywood has long been fascinated with idea, from a 2000 episode of NBC's The West Wing to the current ABC series actually entitled Designated Survivor, in which Kiefer Sutherland plays a Cabinet member who assumes the presidency after a terrorist attack on a State of the Union address wipes out the rest of the other possible successors.